Kelvin is an active member of the Portuguese community. After one of our members asked for a tool to easily query DOT balances on a given day (for tax purposes), he decided to build one. He came up with PolkaHistory, a straight-forward tool to search of your balance on any given day and time. In Brazil, we have to declare the amount of tokens we had in the last day of the last year, this tool makes it very simple.
He also did a version for Kusama at https://www.polkahistory.com/kusama. If you click on the top right corner icon you can switch from Polkadot to Kusama and vice-versa. The application changes from dark mode to white mode when switching networks, pretty cool detail. I'm opened this tip on Polkadot, but I'm also opening here beucase I think he deserves both.
You can verify Kelvin's address on the right side of the page's footer.